Internet Engineering Task Force (IETF) S. Turner
Request for Comments: 6149 IECA
Obsoletes: 1319 L. Chen
Category: Informational NIST
ISSN: 2070-1721 March 2011
MD2 to Historic Status
Abstract
This document retires MD2 and discusses the reasons for doing so.
This document moves RFC 1319 to Historic status.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6149.
Copyright Notice
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Turner & Chen Informational [Page 1]

RFC 6149 MD2 to Historic Status March 20111. Introduction
MD2 [MD2] is a message digest algorithm that takes as input a message
of arbitrary length and produces as output a 128-bit "fingerprint" or
"message digest" of the input. This document retires MD2.
Specifically, this document moves RFC 1319 [MD2] to Historic status.
The reasons for taking this action are discussed.
[HASH-Attack] summarizes the use of hashes in many protocols and
discusses how attacks against a message digest algorithm's one-way
and collision-free properties affect and do not affect Internet
protocols. Familiarity with [HASH-Attack] is assumed.
2. Rationale
MD2 was published in 1992 as an Informational RFC. Since its
publication, MD2 has been shown to not be collision-free [ROCH1995]
[KNMA2005] [ROCH1997], albeit successful collision attacks for
properly implemented MD2 are not that damaging. Successful pre-image
and second pre-image attacks against MD2 have been shown [KNMA2005]
[MULL2004] [KMM2010].
3. Documents that Reference RFC 1319
Use of MD2 has been specified in the following RFCs:
Proposed Standard (PS):
o [RFC3279] Algorithms and Identifiers for the Internet X.509 Public
Key Infrastructure Certificate and Certificate Revocation
List (CRL) Profile.
o [RFC4572] Connection-Oriented Media Transport over the Transport
Layer Security (TLS) Protocol in the Session Description
Protocol (SDP).
Informational:
o [RFC1983] Internet Users' Glossary.
o [RFC2315] PKCS #7: Cryptographic Message Syntax Version 1.5.
o [RFC2898] PKCS #5: Password-Based Cryptography Specification
Version 2.0.
o [RFC3447] Public-Key Cryptography Standards (PKCS) #1: RSA
Cryptography Specifications Version 2.1.
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RFC 6149 MD2 to Historic Status March 2011
Experimental:
o [RFC2660] The Secure HyperText Transfer Protocol.
There are other RFCs that refer to MD2, but they have been either
moved to Historic status or obsoleted by a later RFC. References and
discussions about these RFCs are omitted. The exceptions are:
o [RFC2313] PKCS #1: RSA Encryption Version 1.5.
o [RFC2437] PKCS #1: RSA Cryptography Specifications Version 2.0.
4. Impact on Moving MD2 to Historic
The impact of moving MD2 to Historic on the RFCs specified in Section3 is minimal, as described below.
Regarding PS RFCs:
o MD2 support in TLS was dropped in TLS 1.1.
o MD2 support is optional in [RFC4572], and SHA-1 is specified as the
preferred algorithm.
o MD2 is included in the original PKIX certificate profile and the
PKIX algorithm document [RFC3279] for compatibility with older
applications, but its use is discouraged. SHA-1 is identified as
the preferred algorithm for the Internet PKI.
Regarding Informational RFCs:
o The Internet Users' Guide [RFC1983] provided a definition for
Message Digest and listed MD2 as one example.
o PKCS#1 v1.5 [RFC2313] stated that there are no known attacks
against MD2. PKCS#1 v2.0 [RFC2437] updated this stance to indicate
that MD2 should only be supported for backward compatibility and to
mention the attacks in [ROCH1995]. PKCS#1 [RFC3447] indicates that
support of MD2 is only retained for compatibility with existing
applications.
o PKCS#5 [RFC2898] recommends that the Password-Based Encryption
Scheme (PBES) that uses MD2 not be used for new applications.
o PKCS#7 [RFC2315] was replaced by a series of Standards Track
publications, "Cryptographic Message Syntax" [RFC2630] [RFC3369]
[RFC5652] and "Cryptographic Message Syntax (CMS) Algorithms"
[RFC3370]. Support for MD2 was dropped in [RFC3370].
Turner & Chen Informational [Page 3]

RFC 6149 MD2 to Historic Status March 2011RFC 2818, "HTTP Over TLS", which does not reference MD2, largely
supplanted implementation of [RFC2660]. [RFC2660] specified MD2 for
use both as a digest algorithm and as a MAC (Message Authentication
Code) algorithm [RFC2104]. Note that this is the only reference to
HMAC-MD2 found in the RFC repository.
5. Other Considerations
MD2 has also fallen out of favor because it is slower than both MD4
[MD4] and MD5 [MD5]. This is because MD2 was optimized for 8-bit
machines, while MD4 and MD5 were optimized for 32-bit machines. MD2
is also slower than the Secure Hash Standard (SHS) [SHS] algorithms:
SHA-1, SHA-224, SHA-256, SHA-384, and SHA-512.
6. Security Considerations
MD2 is different from MD4 and MD5 in that is not a straight Merkle-
Damgaard design. For a padded message with t blocks, it generates a
nonlinear checksum as its t+1 block. The checksum is considered as
the final block input of MD2.
As confirmed in 1997 by Rogier et al. [ROCH1997], the collision
resistance property of MD2 highly depends on the nonlinear checksum.
Without the checksum, a collision can be found in 2^12 MD2
operations, while with the checksum, the best collision attack takes
2^63.3 operations with 2^50 memory complexity [MULL2004], which is
not significantly better than the birthday attack.
Even though collision attacks on MD2 are not significantly more
powerful than the birthday attack, MD2 was found not to be one-way.
In [KMM2010], a pre-image can be found with 2^104 MD2 operations. In
an improved attack described in [KMM2010], a pre-image can be found
in 2^73 MD2 operations. Because of this "invertible" property of
MD2, when using MD2 in HMAC, it may leak information of the keys.
Obviously, the pre-image attack can be used to find a second pre-
image. The second pre-image attack is even more severe than a
collision attack to digital signatures. Therefore, MD2 must not be
used for digital signatures.
Some may find the guidance for key lengths and algorithm strengths in
[SP800-57] and [SP800-131] useful.
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RFC 6149 MD2 to Historic Status March 20117. Recommendation
Despite MD2 seeing some deployment on the Internet, this
specification recommends obsoleting MD2. MD2 is not a reasonable
candidate for further standardization and should be deprecated in
favor of one or more existing hash algorithms (e.g., SHA-256 [SHS]).
RSA Security considers it appropriate to move the MD2 algorithm to
Historic status.
It takes a number of years to deploy crypto and it also takes a
number of years to withdraw it. Algorithms need to be withdrawn
before a catastrophic break is discovered. MD2 is clearly showing
signs of weakness, and implementations should strongly consider
removing support and migrating to another hash algorithm.
8. Acknowledgements
We'd like to thank RSA for publishing MD2. We'd also like to thank
all the cryptographers who studied the algorithm. For their
contributions to this document, we'd like to thank Ran Atkinson,
Alfred Hoenes, John Linn, and Martin Rex.
9. Informative References
[HASH-Attack] Hoffman, P. and B. Schneier, "Attacks on Cryptographic
Hashes in Internet Protocols", RFC 4270, November 2005.
[KMM2010] Knudsen, L., Mathiassen, J., Muller, F., and Thomsen,
S., "Cryptanalysis of MD2", Journal of Cryptology,
23(1):72-90, 2010.
[KNMA2005] Knudsen, L., and J. Mathiassen, "Preimage and Collision
Attacks on MD2", FSE 2005.
[MD2] Kaliski, B., "The MD2 Message-Digest Algorithm", RFC1319, April 1992.
[MD4] Rivest, R., "The MD4 Message-Digest Algorithm", RFC1320, April 1992.
[MD5] Rivest, R., "The MD5 Message-Digest Algorithm", RFC1321, April 1992.
[MULL2004] Muller, F., "The MD2 Hash Function Is Not One-Way",
ASIACRYPT, LNCS 3329, pp. 214-229, Springer, 2004.
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